Hip prosthesis comprising a shaft to be inserted into the femur

ABSTRACT

A hip prosthesis includes a straight shaft which is to be inserted into the femur, the straight shaft including a proximal part that is configured to be inserted in the metaphyseal region of the femur. The proximal part has, on each of its front and rear faces, at least one projecting fin with a steep medial flank that deviates at least over part of its length away from the longitudinal direction of the shaft. At its top the proximal part is inclined toward the femoral neck. In this way, when the shaft is driven into the bone, the bone substance surrounding the fin is compacted and is able to absorb greater forces. The height of the fin decreases from its medial flank toward its lateral edge.

For anchoring a prosthesis shaft in the femur, the conditions in themetaphyseal region are different than those in the diaphyseal region. Inthe metaphyseal region, i.e. principally in the region above the lessertrochanter, the wide bone space is filled with spongy bone substance inwhich an artificial receiving channel has to be created to receive aprosthesis shaft. Since the spongy bone substance is soft, thepossibilities of force transmission in this region are limited. Belowthe lesser trochanter, the narrower medullary space is delimited bythick cortical bone, which affords much better conditions for forcetransmission. Prosthesis shafts have therefore been developed that areespecially suitable for anchoring and wedging in this diaphyseal regionof the bone (Schneider: Die Totalprothese der Hüfte [The total hipprosthesis], page 214 et seq.). Their reliable anchoring in thediaphyseal region of the bone has the effect that the metaphyseal regionof the bone is not loaded. If bone substance is not subjected toloading, it is gradually broken down. This is undesirable.

In prostheses which are primarily to be anchored in the metaphysealregion of the bone, it is known (EP-B-761183; EP-A-780106; EP-A-1070490;EP-B-159462; EP-B-821923; EP-B-112435; DE-C-4428099) to improve theanchoring in the metaphyseal region by means of fins and edgesprojecting from the ventral and dorsal surfaces of the main body of theprosthesis. These fins require a considerable extent in the lateromedialdirection if they are intended for anchoring in bone cement or in ahollow that has been reamed out with the same shape (EP-A-780106;EP-A-1070490; DE-C-4428099). In cementless implantation, however, aparticularly stable postoperative fit of the prosthesis in the bone isachieved if fins are used which, in cementless implantation, cut intothe bone substance and compress it (EP-B-761183). The hollow spaceformed in advance in the bone prior to insertion of the prosthesis shaftis then limited to the volume of the main body of the shaft. To ensurethat the fins do not burst the bone when the shaft is driven in, theyare made narrow and have inclined medial and lateral flanks(EP-B-159462; EP-B-821923; EP-B-761183). This limits their ability totransmit forces.

The invention relates to the type of prosthesis in which anchoring issought primarily in the diaphyseal region of the bone. The object of theinvention is to counteract the degradation of bone caused by inadequateloading in the metaphyseal region.

The solution according to the invention is that, in accordance withclaim 1, the metaphyseal region is provided with additionalforce-transmitting means formed by fins. These fins have a particularshape with a steep flank facing in the medial direction. The fin surfacefacing to the front or rear is accordingly made wider than in the caseof the known narrow fins with inclined medial and lateral flanks.However, to ensure that no force arises which could burst the bone whenthe shaft is being driven into said bone, it is further provided thatthe fin decreases in height in the lateral direction from the edgedelimiting the medial flank. The customary, laterally oriented flank ofthe fins thus disappears or is reduced to a relatively low height whichis at most half the height of the medial flank. This is acceptablebecause the forces to be transmitted in the lateral direction by the finare less than the medially directed forces.

The feature to the effect that the medial flank is steep signifies thatit forms an almost right angle with the mediolateral plane of the shaft.It should preferably not deviate from this by more than 25°, morepreferably by not more than 15°.

At least over part of its length, the medial flank deviates from thelongitudinal direction of the shaft and at the top is inclined towardthe femoral neck. This shape has the advantage that when the prosthesisis inserted, if the direction of insertion coincides with thelongitudinal direction of the prosthesis, the medial flank of the fin,acting like a wedge, compacts the spongy bone substance located in frontof it in the insertion direction so that this is able to transmitgreater forces. This effect is primarily produced if the angle enclosedby this flank of the fin and the longitudinal direction of the shaft isbetween 5 and 15°, preferably about 10°.

According to the invention, a corresponding compression on the anteriorand posterior faces of the fins can be achieved by the fact that theheight of the fins above the respective surface of the main body of theshaft increases from the bottom upward. The fin is thus wedge-shaped intwo directions, namely in the first instance toward the medial face andin the second instance toward the anterior or posterior face. The heightof the fin can gradually decrease in the lateral direction, i.e. at theside directed away from the steep flank.

The compression of the spongy bone substance by the fins assumes thatbone substance was previously present at the place where the fins arelocated after implantation. If, before insertion of the prosthesis, achannel for receiving the prosthesis shaft is artificially created inthe metaphyseal region of the bone, this channel should simplycorrespond to the cross-sectional shape of the main body of theprosthesis shaft and should therefore not have any bulged-out areas forsubsequently receiving the fins. If a rasp is used to form thisreceiving channel, its shape ought therefore to correspond only to themain body of the shaft, without having means for removing material inthe fin area. Alternatively, it is also possible to equip the rasp withfins which correspond to the fins of the prosthesis shaft and aredesigned for compression of the bone substance without removingmaterial. The above-described compression of the bone substance in theforce transmission region of the fins thus comes about through the raspitself or is prepared partially by the latter.

The invention is explained in more detail below with reference to thedrawing which depicts an advantageous illustrative embodiment and inwhich:

FIG. 1 shows a front view;

FIG. 2 shows a medial view;

FIG. 3 shows a top view; and

FIGS. 4 to 6 show cross sections through the shaft at the correspondingheights thereof.

The prosthesis is composed of a shaft 1, a neck 2 and a cone 3 forattachment of an articulation head 4 whose circumference is indicated bya dot-and-dash line. The shaft is made up of a proximal portion 6 and adistal portion 7. The proximal portion is elongate in cross section inthe LM direction, as is shown in FIGS. 4 and 5. It is provided with apair of fins 8 for transmitting force to the spongy bone substancesurrounding the shaft in the epiphyseal region of the femur. The shaft 1can be seen as a straight shaft. This means that it is of straightconfiguration and, accordingly, has to be driven into the femur in itslongitudinal direction and in the longitudinal direction of the femoraldiaphysis.

The transition 9 between the proximal portion and the distal portion ofthe shaft is arranged so that, in the implanted state, it comes to lieapproximately at the lesser trochanter, preferably slightly below thelatter, and the distal shaft portion 7 accordingly lies in a region ofthe medullary canal in which the latter is delimited by thick corticalbone. The transition does not need to be specially marked on theprosthesis. It is determined by the fact that it lies at the pointwhere, in the implanted state, the lesser trochanter or preferably thelower edge of the latter is assumed to be located. It lies generallyabout 7 to 9 cm deeper than the center point 5 of the articulation head4, measured according to arrow 10 in the shaft direction.

The distal shaft portion is designed such that it is suitable foranchoring in the diaphyseal region of the femur. To obtain a firm fit,its shaft core is weakly conical and equipped with longitudinal ribs. Inthis way, the bone substance located in the space between the surface ofthe shaft core 15 and the cortical margin of the medullary space, iscompressed, and, in the process, is held firmly by the ribs. The distalportion can also be configured in another way suitable for primaryanchoring of the shaft in the diaphysis of the bone.

The fins 8 rise from the anterior and posterior surfaces 21 of the mainbody 22 of the shaft. They have a steep flank 23 oriented in the medialdirection, and an anterior or posterior surface portion 24 delimitedlaterally by an edge 25. The surface 24 drops off toward the surface 21as its distance from the flank 23 increases, so that the fin acquires anapproximately triangular or trapezoidal cross-sectional shape, as can beseen in FIGS. 3 and 4. Its height above the surface 21 of the main body22 is at least twice as great at the medial edge 23 as it is at thelateral edge.

The fins 8 begin at the transition 9 between the proximal portion andthe distal portion 7 of the shaft, with a zero height and a minimalwidth. In the upward direction, they grow uniformly to their maximumheight and width, which they reach at the upper end 26. With thelongitudinal axis 27 of the shaft, the flank 23 encloses an angle αwhich, in the illustrative embodiment, is approximately 8°. The heightof the flank 23 at the upper end 26 of the shaft is between 2 and 4 mm,preferably about 3 mm. The height of the flank 25 is between zero andhalf the height of the flank 23. In side view, the flank 25 coincideswith the longitudinal axis 27 or extends parallel thereto or at a verysmall angle thereto.

The cross-sectional surface of the fins increases in a wedge shape fromthe bottom upward in two directions, namely toward the flank 23 andtoward the anterior and posterior surfaces 24. If the hollow formed forreceiving the prosthesis shaft in the metaphyseal, spongy region of thefemur is identical in cross section to the main body 22 of the shaft,then the fins 8, when the shaft is driven in, displace the spongysubstance located there and compact it. It is thus made more suitablefor transmitting forces. The dropping-away of the fin from the medialedge to the lateral edge has the further advantage that the spaceavailable in the metaphysis of the bone can be better utilized for alarge shaft design.

The illustrative embodiment shows a linear course of the fins 8. Theirwedge shape, however, can also have a nonlinear course.

Although the prosthesis is intended for primary anchoring in thediaphysis, the fins 21, and the other surfaces 28 of the prosthesisshaft that are oriented in the medial direction, contribute to thetransmission of forces in the metaphyseal region too. The metaphysis ofthe bone thus participates in the transmission of force. The danger ofits degeneration is thus reduced. The long-term secure hold of theprosthesis in the bone is improved.

1. A hip prosthesis comprising a shaft configured to be inserted into afemur and having a proximal part configured to be inserted in ametaphyseal region of the femur which comprises projecting fins on frontand rear faces of the proximal part, each fin having a steep medialflank that deviates away from a longitudinal direction of the shaft andat the top is inclined toward the femoral neck (2), the height of thefins decreasing in a lateral direction from an edge delimiting the steepflank.
 2. The prosthesis as claimed in claim 1, wherein the fins extendrectilinearly at an angle of 5 to 15° with respect to the longitudinaldirection of the shaft.
 3. The prosthesis as claimed in claim 1 or 2,wherein the height of the fin (8) above the surface (21) of the mainbody (22) of the shaft increases from the bottom upward.
 4. Theprosthesis as claimed in claim 1 or 2, wherein the height of the lateraledge of the fin is not greater than half the height of the medial edge.5. The prosthesis as claimed in claim 1 or 2, further comprising adevice for anchoring the endoprosthesis to a diaphysis.
 6. Theprosthesis as claimed in claim 3, wherein the height of the lateral edgeof the fin is not greater than half the height of the medial edge. 7.The prosthesis as claimed in claim 3, further comprising a device foranchoring the endoprosthesis to a diaphysis.
 8. The prosthesis asclaimed in claim 4, further comprising a device for anchoring theendoprosthesis to a diaphysis.